The present invention relates to temperature resistant thermoset plastic polyimide elastomers and more particularly to thermally stable thermoset plastic polyimide elastomers useful in automotive power transmission belts which are reaction injection molded.
The temperature requirements for engine accessory drive belts have increased so dramatically that conventional elastomers, used to fabricate power transmission belts, are not adequate for tomorrow's automobiles.
The obvious solution would be to incorporate new fabricating technology and materials into a totally new method of manufacture for power transmission belts. This is what led us to RIM processing, short for Reaction Injection Molding. This is not to be confused with the Injection Molding process which is widely used today to make thermoplastic molded parts. The Thermoplastic Injection Molding machine simply melts the plastic and injects it into a cold mold where it hardens and assumes the desired shape of the mold. These plastics can be processed over and over again by remelting, and injecting them into new molds.
The RIM machine meters two chemically reactive liquids, commonly designated as "A" and "B" reactants, in a precise volumetric ratio to a impingement mixer at near sonic velocity. The reacting liquid chemicals are then injected into a low pressure mold before they have had sufficient time to polymerize into a solid plastic. The resulting polymer is a fully crosslinked thermoset plastic of enormous molecular weight that can never be reprocessed again, it will thermally degrade before remelting.
The key to the process is in the word "REACTION", i.e., (Chemical Reaction). The polymer is polymerized, created insitu, in the mold by the spontaneous chemical reaction of two liquid oligomer systems with each other. I use the term oligomer system because neither the A or B side is a completely finished polymer. The A and B components are blended chemical intermediates which are liquid and have no chemical reactive to themselves at the processing temperature. The two oligomer intermediates do, however, react very with each other upon mixing. Typical reaction times of 0.2 to 1 second are not uncommon. Finished, completely cured parts are commonly molded in 1 to 2 minutes.
Polymer technology has not kept pace with RIM technology. There a number of RIM polyurea and polyurethane materials which satisfy the basic physical characteristics required in these applications, namely, tensile strength, flexural modulus, hardness, and elongation. In addition, they have superior dynamic properties over conventional millable gum elastomers such as Neoprene, Hycar. and Hypalon. They do not, however, have the temperature resistance necessary for automotive power transmission belts.
Thermal degradation studies conducted on these elastomers clearly show the thermally weak part to be the urethane or urea linkage. The polymer backbone structures are capable of withstanding much higher temperatures. The polythioether backbones are stable to about 600.degree. F., aliphatic polyethers are stable to about 670.degree. F., and aromatic polyethers are stable at temperatures in excess of 700.degree. F. Oligomeric diamines used to formulate polyureas would satisfy all the requirements for power transmission belts if a more heat resistant chemical linkage could be found. It would also satisfy all of the manufacturing requirements if the new chemical linkage would form insitu by RIM processing. In accordance with the present invention, these oligomeric prepolymers are terminated by a maleimide, itaconimide, citraconimide, triazolinedione, or vinylketone and the conventional linkages of the polyurethane or polyurea have been replaced by a much more stable linkage formed by Michael Addition or Diels Alder reaction with these moieties.
Polymers formed by Michael (nucleophilic) Addition of a bismaleimide or itaconimide are known in the art. White, J.E. et al., "Reactions of Diaminoalkanes with Bismaleimides: Synthesis of Some Unusual polyimides," J. Appl. Poly Sci., 29, 891-99 (1984) discloses that polyimide elastomers can be obtained by reacting diaminoalkanes having flexible backbones with aliphatic and aromatic bismaleimides. Examples of the diaminoalkanes are 1,8-diaminoctane, N,N-dimethyl-1,6-hexanediamine.
I have discovered that these also hold true with the bisitaconimides. In fact I have found that the bisitaconimides behave and perform identically to the bismaleimides, and the resulting 3-methylsuccinimide linkage is much more stable than the succinimide linkage formed by the bismaleimides. ##STR2##
This is due to the location of the double bismaleimide, the bond is located in the ring, and the linkage resulting from the Michael Addition to a primary amine leaves a tertiary hydrogen attached to the succinimide ring. The itaconimide on the other hand has its unsaturated bond between a pendant methyl group and the succinimide ring. On reacting with a primary amine the amine is directed to attach directly to the succinimide ring leaving a pendant methyl group rather than a tertiary hydrogen. The pendant methyl group is far more thermally stable than a tertiary hydrogen and it adds flexibility and resilience to the polymer.
U.S. Pat. No. 3,741,942 to Crivello (1973) teaches a polyimide obtained by reaction of a bismaleimide and a dithiol, however, these polyimides, while temperature resistant, do not have the other physical properties required for use in automotive power transmission belts and there is no disclosure of RIM processing of the polyimides.
Bismaleimides have also been used to crosslink unsaturated rubbers as described in U.S. Pat. No. 2,989,504 to Little (1961), and they have been reacted with diamines by Michael Addition in making fibers and molded articles as described in U.S. Pat. No. 2,818,405 to Kovacic (1957), U.S. Pat. No. 3,658,764 to Lyon (1972). U.S. Pat. No. 3,767,626 to Bron (1973), and U.S. Pat. No. 3,878,172 to Bargain et al. (1975), and RE 29,316 to Bargain et al. (1976).
U.S. Pat. No. 3,738,967 to Crivello (1973) teaches that polyimides can also be prepared by a nucleophilic addition reaction of a bismaleimide and hydrogen sulfide. These polyimides are disclosed as being useful in molding, insulation, and coating. Another class of polyimide is obtained by reacting a bismaleimide with a diamine and then a sulfide or dithiol according to U.S. Pat. No. 3,766,138 to Crivello (1973).